1. Field of the Invention
This invention relates to a heavy duty pneumatic radial tire having an improved belt durability more particularly it relates to a heavy duty pneumatic radial tire capable of advantageously attaining resistance to cut separation and resistance to cut penetration in a tread portion.
2. Description of the Related Art
When tires for heavy duty vehicles are run on roads partially including rough road, bad road or roads scattered with foreign objects, they are subjected to the potential of deep cut damage in an outer portion of the tread or tread rubber. The conditions also tend to cause a trouble known as cut separation failure. The term "cut separation failure" used herein means a problem when the tire is subjected to deep cut damage arriving at the belt located in a central region of the tread portion and grows to separation from the cut damaged portion along the outer peripheral surface of the belt during the running of the tire.
Cut separation failure results from the action of shearing strain produced between the outer peripheral surface of the belt and the tread rubber contacting therewith accompanied with the bending deformation behavior of the belt in the peripheral direction of the tread (hereinafter referred to as the circumferential direction) at stepping-in side and kicking-out side of the tread portion during running of the tire. Since the cut separation failure is created between the tread rubber and the outer peripheral surface of the belt, it is a serious problem because the tire is removed from the vehicle at a time when the separation region is remarkably enlarged or tread rubber is sometimes peeled off at the separation region.
To this end, it has been attempted to enhance the bending rigidity of the belt in the circumferential direction for controlling the shearing strain as far as possible. In fact, it is attempted to make an inclination angle of cords, particularly steel cords in the belt with respect to the circumferential direction small as far as possible.
However, the above attempt does not particularly exhibit the remarkable improving effect on the resistance to cut separation. This is because, another separation failure is apt to be caused at the end of the belt as the inclination cord angle becomes smaller. Considering this fact, making the inclination cord angle small to improve the bending rigidity of the belt in the circumferential direction is naturally critical and hence it is difficult to develop the expected excellent resistance to cut separation.
Furthermore, when the inclination cord angle is made small to enhance the bending rigidity of the belt as a whole, an enveloping property (EP) in the riding of the tread portion over projections such as rock or stone or EP property against projections is deteriorated. A chance of subjecting the tread portion to cut damage increases but also the cut depth becomes deeper, which sometimes brings about a fatal failure of passing the cut damage through the entire tread portion or so-called cut penetration failure.